Should I update to that one and then disable this override again? I don’t understand what is the normal process here?
Eventually Chum started working again on its own, so it appears like the normal process is just to wait a long time. It should not be neccessary to do any manual steps after updating to a new big Sailfish release.
Yes.
I don’t understand what is the normal process here?
For EA releases you may have to use the version override of the SailfishOS:ChumGUI app set to the latest non-EA release of SailfishOS, if you want to continue to use SailfishOS:Chum.
Or you just wait, until the new release has its corresponding backing repository at SailfishOS:Chum.
This is a case of RTFM, for details see there.
It is not, if you are patient and can live without using SailfishOS:Chum for a while. And all this is well documented behaviour. Mind that you are all using an EA release, which is for testing purposes only: This is what you were doing here.
Many thanks to all testers who report(ed) back constructively!
I am also on the EA release and just want to report that today the repository is finally up. The one app I use Chum for is still not up, but the repo is working as intended.
I knew what EA is and was patient. I made no changes to the application configuration and „the problem fixed itself”. Thank you for your work, Chum maintainers.
I’ve gotten around to making a bug report now, it’s my first one yet, so hopefully I didn´t screw up 
Everything has automatically started working fine now, perhaps an automated update or something, but for me all issues are solved 